3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
FIG. 1 illustrates a radio access network in an LTE system. An eNB 101a serves a UE 103 located within the RBS's geographical area of service or the cell 105a. The eNB 101a is directly connected to the core network. The eNB 101a is also connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. Although the eNBs of this example network serves one cell each, an eNB may serve more than one cell.
Release 8 of LTE supports uplink MU-MIMO, which implies uplink transmissions from multiple UEs using the same uplink time-frequency resource and relying on the availability of multiple receive antennas at the RBS to separate the two or more transmissions. The difference between ordinary Frequency Division Multiplexing (FDM) scheduling and MU-MIMO scheduling is schematically illustrated in FIG. 2. In the upper part of FIG. 2, all UEs (UE1, UE2, UE3, UE4) are allocated different resource blocks in frequency, also referred to as FDM scheduling. In the lower part of FIG. 2, MU-MIMO scheduling is illustrated, where UE1 and UE2 are co-scheduled on the same resources in frequency, and UE3 and UE4 are co-scheduled on the same resources.
One important benefit of uplink MU-MIMO is that it can get similar gain in system throughput as Single User (SU)-MIMO where spatial multiplexing is used, without the need for multiple transmission antennas at the UE side. MU-MIMO thus allows for a less complex UE implementation. The potential system gain of uplink MU-MIMO relies on more than one UE being available for transmission using the same time-frequency resource. The process of pairing UEs that should share the same time-frequency resources is non-trivial and requires suitable radio-channel conditions.
Ideally, UEs that are paired, i.e., the UE group size is two, should have orthogonal or almost orthogonal channels, so that they cause as little interference as possible to each other. If the two signals can be perfectly separated at the receiver, and both signals are transmitted with the same power as in the single UE case, there is a potential for a 100% cell or UE throughput gain without power increase. However, the radio channel of the paired UEs are seldom ideally orthogonal to each other, which means the signal of one paired UE may contribute with a relatively large interference to the other one. Thus the interference that one UE experiences after being paired with another UE using MU-MIMO scheduling may be increased quite much compared to if the UEs are not paired, and thus are not MU-MIMO scheduled. Similarly, the interference that one UE experiences after being scheduled in normal FDM may be decreased quite much compared to when the UE is scheduled in pair with another UE. Therefore, MU-MIMO scheduling may cause an abrupt Signal to Interference and Noise Ratio (SINR) variation, which is illustrated in the three graphs in FIG. 3. The upper left graph, 303, illustrates the uplink bit rate in kilobits per second (kbps) over time for a cell. The lower left graph, 304, illustrates the SINR for the Physical Uplink Shared Channel (PUSCH) in dB over time for the first UE with a Radio Network Temporary Identifier (RNTI) equal to 242, and the right hand graph, 305, illustrates the SINR for the PUSCH in dB over time for a second UE with a Radio Network Temporary Identifier (RNTI) equal to 134. When the first and the second UE switch from non-MU-MIMO scheduling to MU-MIMO scheduling in pair with each other, which happens at a time indicated by the broken line 301 in all three graphs, the uplink bit rate of the cell increases from around 18000 kbps to around 36000 kbps while the first and the second UEs' SINR are abruptly decreased. This means that the two UEs' transmission power should be increased accordingly to meet the SINR or SINR target requirement. Analogously, the UEs' SINR increase abruptly when the first and second UEs switch from MU-MIMO scheduling in pair to a de-paired non-MU-MIMO scheduling, which happens at a time indicated by the broken line 302 in all three graphs. At de-pairing, the UEs' transmission power should be decreased accordingly in order to generate less interference and to decrease the power consumption by this UE.
The specified power control step size for uplink transmission power control is given by [−1, 0, 1, 3] dB, meaning that the maximum step size is minus 1 dB when the power is to be decreased, and plus 3 dB when the power is to be increased for a UE. In each Round Trip Time (RTT), which corresponds to approximately 5 milliseconds (ms), the power may thus at the most be increased by 3 dB or decreased by 1 dB using transmission power control commands. However, the difference between MU-MIMO and non-MU-MIMO SINR in the switch instant is quite large as exemplified with the field test results shown in the graphs of FIG. 3. Therefore it will take quite some time for the power control to follow the abrupt SINR variation. As may be seen in the graphs of FIG. 3, the SINR variation may be up to 15 dB. With a step size of +3 dB, it would take 5 RTT or 25 ms to adapt the power to the SINR change. Such an abrupt interference or SINR variation may also happen when the scheduler in the RBS changes the partner of one paired UE, e.g. due to changes of radio channel orthogonality between different UEs.
There are currently three different scheduling schemes with different complexity applied for MU-MIMO scheduling:                1. Static scheduling, i.e. the UEs are randomly divided into pairs of two UEs. The pairs persist as long as all UEs remain active.        2. Island scheduling, i.e. UEs are paired with each other only if both of them have a larger estimated throughput compared to non-MU-MIMO scheduling. The estimated throughput is based on an estimated SINR which takes the interference from the other paired UE into account.        3. Proportional Fair in Time and Frequency (PFTF) scheduling, i.e. UEs are paired with each other on resource blocks in which they may have the largest throughput. The scheduling thus considers frequency selectivity in addition to the considerations in scheduling scheme 2 above.        
The drawback of scheduling scheme 1 is that the interference between MU-MIMO UEs is not considered when deciding to pair the UEs. The UEs could be paired with each other using MU-MIMO scheduling, even when the decision results in a cell or UE throughput loss compared to non-MU-MIMO scheduling.
The drawback of scheme 2 and 3 is that a UE will experience abrupt interference and SINR variation quite often, as UEs frequently get paired or de-paired or changes their MU-MIMO pair partner. Since power control and/or SINR measurements cannot follow this abrupt SINR quickly enough, the link adaptation may be seriously affected. The link adaptation deterioration may finally result in both a UE and a cell performance degradation.